Nondestructive evaluation techniques have been developed that can identify diffusion bonds that contain chemical contamination of the bonding interfaces, or arrays of pores resulting from incomplete bonding. U.S. Pat. No. 4,173,139 describes calibration standards for ultrasonic flaw detection of diffusion bonds that contain voids. U.S. Pat. No. 4,215,583 describes an apparatus and associated method of identifying disbonded regions in diffusion bonds by the reflection of sound from such flaws. It is known that disbonds can be imaged by techniques such as acoustic microscopy, that employ high frequency (140 MHz) sound energy. Lower frequency ultrasonic energy, such as that used in well-known C-scan evaluation techniques, may also be used to detect voids, and the reflection data from bonds containing voids has been related to the tensile strength of the bonds.
It has been observed that even when diffusion bonds are formed that do not contain voids or chemical contamination when evaluated using various ultrasonic and other known materials evaluation techniques, including destructive techniques such as optical and SEM metallography, that it is possible to have bonds that have significantly reduced, or non-optimum, mechanical properties, such as tensile and shear strength, than the adjacent materials between which the bond is formed. This difference in mechanical properties appears to be largely related to the dgree to which diffusion processes have prompted grain growth across the interface. If the grains are aligned along the interface, such as in the form of a planar array, the mechanical properties are generally reduced. If grain growth has occurred across the interface, the mechanical properties are generally improved. While useful for the evaluation of voids and chemical contamination at bonded interfaces, known ultrasonic techniques, such as C-scan evaluation, do not positively identify bonded interfaces that are non-optimum, as described further.
Because of the desirability of using ultrasonic nondestructive evaluation (NDE) techniques to detect microstructural features, such as non-optimum bond interfaces as described herein, it is therefore desirable to develop ultrasonic detection and evaluation methods that are capable of identifying these features.